(a) Field of the Invention
The present invention relates to an LCD (liquid crystal display) device and an LCD projector including the LCD device.
(b) Description of the Related Art
An LCD projector includes a light source, and an LCD device used as a light valve having a function for controlling transmittance of the light emitted from the light source. The LCD projector projects an enlarged image of the light transmitted through the LCD device onto a screen. The LCD device has an LC layer, and a pair of polarizing plates having polarization axes perpendicular to each other. The LC layer controls the polarized direction of the incident light to pass through the polarizing plate the light needed to display the image.
The LC layer controlling the polarized direction of the transmitted light is sandwiched between a TFT (thin-film-transistor) substrate and a counter substrate. The TFT substrate includes a transparent substrate mounting thereon an array of TFTs, pixel electrodes each corresponding to one of the TFTs, and drive circuits for driving the array of the TFTs. The counter substrate includes a transparent substrate mounting thereon a counter electrode. The TFT substrate and the counter substrate sandwich therebetween the LC layer so that the surfaces of the TFT substrate and the counter substrate on which respective electrodes are formed oppose each other.
The thicknesses of the TFT substrate and the counter substrate are as small as about 0.5 to 1.2 mm. Thus, even the outer surfaces (i.e., the sides far from the LC layer) of the TFT substrate and the counter substrate are located in the vicinity of the LC layer, on which the transmitted light is focused. This renders a damage or dust, if any, on the outer surface of the substrate to be projected onto the screen as an image, thereby degrading the image quality of the LCD projector. For preventing the image degradation, a pair of dustproof glass plates (or dustproof substrates) are bonded onto the respective substrates, the dustproof glass plates having a thickness of around 0.5 to 1.2 mm, to prevent generation of the damage or attachment of the dust. This thickness of the dustproof glass plate allows the damage or dust on the dustproof glass plate to be well apart from the LC layer, thereby suppressing the influence by the damage or dust on the image quality.
An up-to-date LCD projector is requested to have a higher luminance in view of a bright room in which the image is projected and observed. This requests the light source of the LCD projector to have a higher luminance. In general, the LCD device is heated by the incident light due to absorption thereof, wherein a higher intensity of the incident light causes a higher temperature rise. If a large temperature difference arises between the central area and the peripheral area of the LCD device, a stress is generated within the TFT substrate, counter substrate or dustproof substrate constituting the LCD device. This stress renders the birefringence of the substrate material to generate retardation, wherein the light transmitted at the portion of the substrate applied by the stress has a phase deviation. The retardation, if generated, causes part of the polarized light to pass through the polarized plate upon display of a black color, thereby generating an “undesirable light pass” phenomenon. The undesirable light pass reduces the contrast ratio of the image projected on the screen, thereby degrading the image quality of the LCD projector.
In the mean time, the conventional LCD device includes a TFT substrate made of a quartz glass having a higher heat tolerance because TFTs are formed on the transparent TFT substrate by using a high-temperature polysilicon technique. The quartz substrate has a lower coefficient of thermal expansion (CTE) of 0.56×10−6/K and thus shows a lower retardation caused by a temperature rise. However, the material for the quartz glass is extremely expensive and has a smaller sheet size, which means a smaller number of TFT substrates being obtained from a single sheet of quartz glass. For example, considering that 1-inch LCD devices are to be manufactured from a 6-inch wafer, the number of 1-inch LCD devices obtained therefrom is only 19 at most. This raises the cost of the LCD devices.
In view of the above, a low-temperature polysilicon technique is increasingly employed for manufacturing the LCD device, wherein TFTs are formed at a lower temperature. This means that an inexpensive glass such as non-alkali glass can be used for the TFT substrate. However, the inexpensive glass generally has a higher CTE and thus exhibits a higher retardation caused by the birefringence.
Patent Publication JP-A-2001-042279 describes a technique for suppressing the retardation by using a substrate having a lower CTE. FIG. 10 shows the LCD device described in this publication, which includes an LC layer 104, TFT substrate 102 and counter substrate 103 sandwiching therebetween the LC layer 104, and dustproof substrates 105 and 106 attached onto the outer surfaces of the TFT substrate 101 and the counter substrate 102, respectively. The polarizing plates are disposed apart from the LCD device in an LCD projector and thus not depicted in this figure. The counter substrate 103 mounts thereon a counter electrode and a micro-lens for focusing the incident light. In this structure, each of the TFT substrate 102, counter substrate 103, and dustproof substrates 105 and 106 is made of a glass having a lower CTE, as low as 1×10−6/K or lower in the absolute value thereof.
Patent Publication JP-A-9-113906 describes another technique for suppression of the retardation. FIG. 11 shows the LCD device described in the publication, which includes an LC layer 114, TFT substrate 112 and counter substrate 113 sandwiching therebetween the LC layer 114, heat radiation substrates 115 and 116 attached onto the outer surfaces of the TFT substrate 112 and the counter substrate 113. Each of the heat radiation substrates 115 and 116 is made of a quartz glass or heat-tolerance glass having a coefficient of thermal conductivity (CTC) of not lower than 1 W/m·K. The heat radiation substrates 115 and 116 also act as dustproof substrates. The heat radiation substrates 115 and 116 assist or accelerate heat radiation to reduce the temperature difference between the central area and the peripheral area of the LCD device 111, thereby suppressing the retardation.
Patent Publication JP-A-11-149071 describes a technique for suppressing the retardation by using a compensation substrate having a photoelastic coefficient which has a sign opposite to the sign of those of the TFT substrate and the counter substrate. FIG. 12 shows the LCD device described therein, which includes an LC layer 124, TFT substrate 122 and counter substrate sandwiching therebetween the LC layer 124, and compensation substrate 125 and micro-lens substrate 126 attached onto the outer surfaces of the TFT substrate 122 and the counter substrate 123, respectively. The micro-lens substrate 126 has thereon a micro-lens for focusing the incident light. It is recited in the publication that each of the TFT substrate 122, counter substrate 123 and micro-lens substrate 126 is made of a glass having a positive photoelastic coefficient, whereas the compensation substrate 125 is made of acrylic resin having a negative photoelastic coefficient.
In the configuration of the LCD device described in JP-A-11-149071, upon generation of a temperature difference, the light transmitted through the compensation substrate 125 having a negative photoelastic coefficient has a phase deviation opposite to the phase deviation caused by the TFT substrate 125 and counter substrate 123. That is, the compensation substrate 125 cancels the retardation caused by the TFT substrate 122 and counter substrate 123.
In the LCD device 101 described in JP-A-2001-042279, all the substrates have lower CTEs of not higher than 1×10−6/K for suppression of the stress. This causes a higher cost for the LCD device due to expensive substrate materials. In addition, even the low-CTE glass cannot sufficiently reduce the retardation, whereby there remains some retardation in the respective substrates. A larger number of the substrates cause larger cumulative retardation, thereby causing degradation of the image quality due to the undesirable light pass. In particular, the degradation of the image quality is more noticeable in the case of a higher-luminance LCD projector having a luminance of 3000 ANSI lumen.
In the LCD device 111 described in JP-A-9-113906, a low-cost LCD device can be obtained by using an inexpensive glass for the TFT substrate. However, the heat radiation efficiency of the quartz glass or heat-tolerance glass used as the heat radiation substrates is not sufficient in the case of the glass having a higher CTE, unable to sufficiently suppress the undesirable light pass caused by the retardation. In particular, the degradation of the image quality is more noticeable in the case of a higher-luminance LCD projector having a luminance of 3000 ANSI lumen.
In the LCD device 121 described in JP-A-11-149071, the materials having negative photoelastic coefficients are limited, whereby the design choice for the substrate material is narrow. This impairs reduction of the cost for the substrate material and thus provision of a lower-cost LCD device.